When the hashCode method is invoked on an object repeatedly during an execution of an application, it must consistently return the same value, provided no information used in equals comparisons is modified.
Hmm, something I haven't questioned before: If information in the object does change that changes its equals & hashcode, a `get` on the new value will fail, right? Because the object will still be in the old hashcode's bucket. I wonder how much this has actually bitten people...
